could be it, you should try D-Trogh's way of making tilesets/maps using sphere. It always worked out great for me.
EDIT:
I tried to insert a REALLY big background image today, and it still turned out fine. So I really don't know what causes the problem, here take a look:
You should Include a text part in your tut btw, I can help you by making a nintendo styled text(like above)mini tut with it.
Don't mind the kingdra under the logo, Still need to edit that.